Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments in securities:
Investments in trading securities	$ 51,324	$ 56,485
Total financial instruments	51,324	56,485
Level 1
Investments in securities:
Investments in trading securities	48,998	53,319
Total financial instruments	48,998	53,319
Level 2
Investments in securities:
Investments in trading securities	0	0
Total financial instruments	0	0
Level 3
Investments in securities:
Investments in trading securities	0	0
Total financial instruments	0	0
Inputs, Net Asset Value [Member]
Investments in securities:
Investments in trading securities	2,326	3,166
Total financial instruments	$ 2,326	$ 3,166